1. Field of the Invention
This invention relates to a piezoelectric actuator used as a tracking actuator for a magnetic disk apparatus and a head assembly which uses the piezoelectric actuator.
2. Description of the Related Art
In recent years, reductions in the size and thickness of a magnetic disk apparatus which is a kind of external storage apparatus for computers has been and is proceeding, and further reduction in power consumption is demanded. Also, increases in recording density and capacity of magnetic disk apparatuses is demanded. An increase in the capacity of a magnetic disk apparatus can be achieved generally by increasing the recording capacity per disk. However, if the recording capacity is increased without changing the diameter of the disk, then the track pitch decreases. Therefore, it is a technical problem how to achieve accurate positioning of a head element which performs reading out from and writing on a recording track, and a head actuator having a high degree of positioning accuracy is demanded.
Conventionally, in order to achieve high accuracy head positioning in a magnetic disk apparatus, generally it has been attempted to enhance the rigidity of a movable part such as an actuator arm to raise the main resonance point frequency in the in-plane direction. However, there is a limitation to the enhancement of the resonance point, and even if the in-plane resonance point of the movable part can be raised by a great amount, the problem that vibration originating from the spring characteristic of a bearing which supports the movable element is generated and deteriorates the positioning accuracy cannot be solved.
As one of the countermeasures for solving such problems, a so-called double actuator wherein a second actuator for track following, that is, a tracking actuator, is mounted at an end of an arm of a head actuator. The tracking actuator moves the head provided at an end portion of the arm by a very small amount independently of movement of the head actuator to achieve tracking of the head.
Such tracking actuators are roughly divided into three types. According to the first type, the tracking actuator is provided between the end of the actuator arm and a suspension. According to the second type, the tracking actuator is provided between a gimbal formed at an end of the suspension and the head slider. According to the third type, only a magnetic head element (electromagnetic transducer) formed integrally on the head slider is moved.
The three types of tracking actuators (microactuators) individually have advantages and disadvantages. The first type is advantageous in that, because the tracking actuator section is comparatively large and an expansion mechanism can be used at an end of the head, the displacement of the tracking actuator section need not be set very great. However, the first type has a limitation whenever the resonance frequency is attempted to be raised. The third type can achieve a considerably high resonance frequency. However, the third type involves difficulty in the production process and requires production of displacement by a great amount by means of a tracking actuator. The second type has intermediate features.
A head assembly having a tracking actuator of the second type is disclosed in Japanese Patent Laid-Open No. Hei 11-273041. In the head assembly, two piezoelectric actuators extend in parallel to each other toward a head slider from a common first fixed member secured to a suspension. An end of each of the piezoelectric actuators is connected to a common second fixed member on the head slider. The head slider can make a rocking motion around the first fixed member based on expansion and contraction of the piezoelectric actuators. In particular, when the piezoelectric actuators expand and contract, the head slider can displace by a very small distance in a radial direction of, for example, a magnetic disk. As a result, the magnetic head element on the head slider can continue to follow up a recording track on the magnetic disk with a high degree of accuracy.
The characteristics required for the tracking actuator of the second type are that the center axis of the slider and the axis of pivotal motion of the tracking actuators coincide with each other, that the amount of movement is 1 μm or more when a voltage equal to or lower than 30 V is applied and that the in-plane resonance frequency is 20 kHz or more. A head assembly which includes a tracking actuator which satisfies the characteristics described above has been proposed by the assignee of the present application (Japanese Patent Application No. 2001-318985). In the head assembly of the preceding application, a pair of piezoelectric actuators are disposed in parallel to each other, and the piezoelectric actuators are contracted to pivot a slider in one direction to achieve tracking. For the power supply, it is necessary to form driving wirings for the two piezoelectric actuators and wirings for the head element on the suspension. From the point of view of securing of the space for wirings and the degree of freedom in design, it is preferable to minimize the number of wirings to be formed on the suspension. Further, in order to lead out electrodes from a portion which is not secured to the suspension, it is necessary for the electrode leading out portion to have a structure which does not disturb deformation of the piezoelectric actuators when a voltage is applied.
Usually, a multi-layer piezoelectric element is used for a piezoelectric actuator. A multi-layer piezoelectric element is individually cut out from a piezoelectric substrate obtained by baking a multi-layer green sheet using a dicing saw or the like. In the case of a piezoelectric actuator used for very small displacement of a magnetic head of a magnetic disk apparatus, the thickness of each layer is several tens of μm or less in order to obtain a desired displacement amount or from a restriction in dimension. Therefore, if metal sag upon cutting of the piezoelectric substrate is produced, there is the possibility that the sag may cause short-circuiting between electrodes. Further, if Ag—Pd is used for the electrodes, then there is the possibility that use of the electrodes under a high-temperature high-humidity condition may unfavorably give rise to migration of Ag.